Corals, Worms, etc. 



in marshy pools. Stentor may be "chopped, broken or 

 shaken up into pieces of all sizes and shapes, and every 

 piece, provided only that it is above a definite minimum 

 size (less than -^^ inch in diameter, and in bulk only one or 

 two per cent, of a full-grown Stentor] and that it contains a 

 piece of the nucleus, will blossom out as a minute but fully 

 formed individual, which will feed and grow and be indis- 

 tinguishable from a product of natural generation." 



We have just mentioned that when the sea-urchin is 

 attacked by the starfish it defends itself by biting. If 

 we examine an urchin carefully we shall see that its 

 armour consists of two kinds of spines, some relatively 

 long and sharply pointed, others smaller and terminating 

 in a pair of pincers. It is with these small pincers that 

 the urchin attacks its adversary. Their main use is the 

 capture of small prey, and they are also used to clean the 

 larger spines, for, despite the fact that the sea-urchin is a 

 slow-moving creature, it never allows itself to become over- 

 grown with seaweed as do so many animals of a sluggish 

 nature. One, at least, of our common urchins, the purple- 

 coloured individual so common round our coasts, is a 

 burrower of no mean order. In limestone and other 

 rocks it excavates holes often as deep as ten inches. It is 

 not known precisely how the industrious little creature 

 works, but from the fact that the spines of the urchins 

 found in these excavations are always considerably worn 

 it is quite probable that the work is accomplished by the 

 constant movement of the larger spines on their ball-and- 

 socket joints. 



The urchins' hollows make good shelters for their 

 inhabitants ; for, although the urchin can walk slowly by 

 a movement of his spines, he has not the power of clinging 

 firmly to his rocky home. Therefore till he has con- 

 structed his shelter he is at the mercy of the waves. The 

 closely related sea-cucumbers are of little interest to us. 

 In one of them dwells a little fish, with what benefit to the 

 sea-cucumber it is hard to say. Another species, under 



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